Attempts have recently been made to improve the combustion efficiency of an engine in order to enhance the fuel efficiency of an automobile. One of the technologies for the improvement of combustion efficiency is an increase in compression ratio. It has already been theoretically certified that an increase in compression ratio leads to an improvement in the heat efficiency of an internal combustion engine. The compression ratio of a gasoline engine is set to a value of about 10, and the compression ratio of a diesel engine is set to a value of about 18. Thus, the diesel engine exhibits higher heat efficiency. With an increase in compression ratio, the gasoline engine becomes more likely to cause abnormal combustion called knocking. Therefore, the increase in compression ratio has a limit.
A technique of re-circulating an exhaust gas (hereinafter referred to as an “Exhaust Gas Recirculation gas” or “EGR gas”) to an intake and re-introducing the gas into a combustion chamber to combust the gas has already been proposed as a technology for preventing the occurrence of knocking. The technique aims at taking in many inert ingredients contained in the EGR gas, such as CO2 and N2, to increase amounts of working mixture which does not contribute to combustion. As a result, a burning reaction becomes slow, which reduces a combustion rate. The technique can prevent the occurrence of knocking even in the engine having a high compression ratio and allows an increase in compression ratio to a value of about 14. Moreover, this technique is applicable also to a high supercharger engine.
On the other hand, the technique for re-introducing and burning the EGR gas has been reported as follows. If the EGR gas is taken in more than a determined amount, combustion failures, such as misfiring and unburning, will occur for reasons of deterioration of ignitability, a decrease in combustion rate, and the like, which will also make variations in combustion greater.
Therefore, in order to increase the compression ratio of the engine by utilization of the EGR gas, it is necessary to detect knocking, or abnormal combustion, and misfiring which induces variations in combustion. As one of means for detecting the abnormal combustion, such as knocking and misfiring, a method for detecting ions generated during combustion as an ion signal is effective. A value of ion signal significantly changes according to operating conditions of the engine (an engine torque, an engine speed, etc.). For this reason, an ion signal value (a threshold value for determination) for determining the abnormal combustion needs to be changed according to the operating conditions. To this end, a mean value of ion signals for several cycles in the past is taken as a background level. A threshold value for determination is computed from the background level. However, when a change occurred in a load of the engine or the engine speed, such a method brings about a problem of a given lag occurring in the changes of the threshold value for determination.
Technology described in connection with Patent Literature 1 has been known as a technique such as that an ion current value (background) acquired during normal combustion is stored on an engine operation map made up of axes representing engine load and engine speed, a threshold value for determining knocking is set on the basis of the value, and an update is made to the threshold value for determination in each operation range by means of learning.